The present invention relates to woven composite forming fabrics for use in papermaking machines. The term xe2x80x9ccomposite forming fabricxe2x80x9d refers to a forming fabric comprising two woven structures, which are the paper side layer and the machine side layer. Each of these layers is woven to a repeating pattern, and the two patterns used may be substantially the same or they may be different; at least one of the patterns includes the provision of binder yarns which serve to hold the two layers together. As used herein, such fabrics are distinct from those described, for example, by Johnson in U.S. Pat. No. 4,815,499 or Barrett in U.S. Pat. No. 5,544,678, which require additional binder yarns, in particular weft yarns, to interconnect the paper and machine side layers. In the composite forming fabrics of this invention, the paper side layer and the machine side layer are each woven to different, but related, weave patterns, and are interconnected by means of the paper side layer warp yarns.
In composite forming fabrics that include two essentially separate woven structures, the paper side layer is typically a single layer woven structure which provides, amongst other things, a minimum of fabric wire mark to, and adequate drainage of liquid from, the incipient paper web. The paper side layer should also maximise planar support for the fibers and other paper forming solids in the paper slurry while providing sufficient open area to allow adequate drainage. The machine side layer is also typically a single layer woven structure, which should be tough and durable, provide a measure of dimensional stability to the composite forming fabric so as to minimize fabric stretching and narrowing, and sufficiently stiff to minimize curling at the fabric edges. It is also known to use double layer woven structures for either or both of the paper and machine side layers.
The two layers of a composite forming fabric are interconnected by means of either additional binder yarns, or intrinsic binder yarns. Additional binder yarns do not contribute significantly to the fundamental weave structure of the paper side surface of the paper side layer, and serve mainly to bind the two layers together. In comparison, intrinsic binder yarns both contribute to the structure of the paper side layer and also serve to bind together the paper and machine side layers of the composite forming fabric. The chosen yarns may be either warp or weft yarns. The paths of the yarns are arranged so that the selected yarns pass through both layers, thereby interconnecting them into a single composite fabric. Examples of prior art composite forming fabrics woven using intrinsic binder warp or weft yarns are described by Osterberg, U.S. Pat. No. 4,501,303; Bugge, U.S. Pat. No. 4,729,412; Chiu, U.S. Pat. No. 4,967,805, U.S. Pat. No. 5,291,004 and U.S. Pat. No. 5,379,808; Givin, U.S. Pat. No. 5,052,448; Wilson, U.S. Pat. No. 4,987,929 and U.S. Pat. No. 5,518,042; Ward et al, U.S. Pat. No. 5,709,250; Vohringer, U.S. Pat. No. 5,152,326; Johansson, U.S. Pat. No. 4,605,585; Hawes, U.S. Pat. No. 5,454,405; Wright, U.S. Pat. No. 5,564,475; and Seabrook et al, U.S. Pat. No. 5,826,627. Additional binder yarns have been generally preferred over intrinsic binder yarns for commercial manufacture of composite forming fabrics because they were thought to be less likely to cause discontinuities, such as dimples, in the paper side surface of the paper side layer. Examples of prior art fabrics woven using additional binder yarns are described by Johansson et al., CA 1,115,177; Borel, U.S. Pat. No. 4,515,853; Vohringer, DE 3,742,101 and U.S. Pat. No. 4,945,952; Fitzka et al, U.S. Pat. No. 5,092,372; Taipale, U.S. Pat. No. 4,974,642; Huhtiniemi, U.S. Pat. No. 5,158,117; and Barreto, U.S. Pat. No. 5,482,567.
In composite forming fabrics where intrinsic warp binder yarns from the machine side layer have been used to interconnect the paper and machine side layers, the prior art has generally advocated modifying the path of the selected machine side layer warps so as to bring these yarns up to the paper side layer to interlace with it at selected weft knuckles. A known disadvantage associated with this practice is that the area immediately adjacent to these tie locations tends to become pulled down into the fabric structure, well below the plane of the adjacent knuckles, causing a deviation in the paper side surface of the paper side layer, commonly referred to as a xe2x80x9cdimplexe2x80x9d. These dimples frequently create a pronounced unevenness in the paper side surface of the fabric, which can result in an unacceptable mark in any paper formed on the fabric. The residual impression made by the weave design of the forming fabric on the side of the paper sheet in contact with the fabric is referred to as xe2x80x9cwire markxe2x80x9d or xe2x80x9cmarkxe2x80x9d.
In comparison, intrinsic weft binder yarns have been found to cause less paper side surface dimpling, and hence have been a preferred method of interconnecting the layers of composite forming fabrics. However, there are a number of problems associated with their use.
First, intrinsic weft binder yarns have been found to cause variations in the cross-machine direction mesh uniformity of the paper side surface of the paper side layer in certain weave patterns, resulting in an unacceptable level of wire mark in some grades of paper.
Second, fabrics woven using intrinsic weft binder yarns are known to be susceptible to lateral contraction, or narrowing, when in use. Lateral contraction may be defined as the degree to which a fabric narrows when machine direction (or longitudinal) tension is applied. If the fabric narrows excessively under this tension, particularly at driven rolls in the forming section, the resulting width changes will cause the fabric to buckle or form ridges. Generally, single layer fabrics, and composite fabrics having additional or intrinsic weft binder yarns, exhibit much higher degrees of lateral contraction than either double layer, or extra-support double layer, fabrics of comparable mesh.
Third, composite forming fabrics containing intrinsic weft binder yarns are less efficient to weave than comparable intrinsic warp binder designs, because a greater number of weft yarns is required to provide a reliable interconnection between the paper side layer and the machine side layer. Comparable fabrics whose designs utilise intrinsic warp binder yarns require fewer weft yarns per unit length, since none of the weft yarns is utilised to interconnect the paper and machine side layers. For example, a fabric containing intrinsic warp binder yarns whose paper side layer is woven so as to provide 31.5 weft yarns/cm, and 15.75 weft yarns/cm on its machine side layer (resulting in a 2:1 ratio of the paper side layer to machine side layer weft yarn count), has a total weft yarn count of 47.25 yarns/cm. A comparable fabric containing intrinsic weft binder yarns, woven at 31.5 weft yarns/cm in its paper side layer, at 15.75 weft yarns/cm in its machine side layer, and which employs additional weft yarns to interconnect the layers, has a total weft yarn count of between 55 to 63 weft yarns/cm, because additional weft yarns must be provided so as to tie the two layers together. Thus, composite forming fabrics that utilise intrinsic warp binder yarns to interconnect their paper and machine side layers require up to 25% fewer weft yarns to weave each unit length, making them more efficient to produce.
Fourth, a fabric utilizing intrinsic warp binder yarns will generally have a lower caliper (and provide a lower void volume) than a comparable fabric of similar specification utilizing intrinsic weft binder yarns. Because there are fewer weft yarns per unit length, those remaining do not contribute as much to the thickness of the fabric.
A benefit provided by composite fabrics utilizing intrinsic warp binder yarns is their increased resistance to delamination, when compared to a composite fabric utilizing either additional or intrinsic weft binder yarns. Delamination, which is the catastrophic separation of the machine and paper side layers, is generally caused by one of two mechanisms. The first is abrasion of the binder yarn where it is exposed on the machine side of the fabric as it passes in sliding contact over the various stationary elements in the forming section. In composite fabrics utilizing intrinsic warp binder yarns, it is possible to recess the warp binder yarns relative to the wear plane of the fabric to a greater degree (e.g. by as much as 0.05-0.076 mm further away from the wear plane) than is possible in a comparable fabric utilizing intrinsic weft binder yarns. This means that more machine side layer warp and weft yarn material must be abraded away from the running side of a fabric utilizing intrinsic warp binder yarns before the tie strands are broken, and the two layers delaminate, than in a comparable fabric utilizing intrinsic weft binder yarns.
The second delamination mechanism, which is encountered more rarely than the first, is that of internal abrasion of the binder yarns between the machine and paper side layers as they flex or shift relative to one another. The presence of abrasive fillers in the stock, such as clay, titanium dioxide and calcium carbonate, greatly exacerbates the rate of this type of abrasion. Composite forming fabrics whose paper and machine layers are well interlaced so as to prevent or reduce relative movement of these layers (such as in the fabrics of the present invention utilizing intrinsic warp binder yarns) will experience less internal abrasion than comparable fabrics utilizing intrinsic weft binder yarns. They are therefore less susceptible to delamination by internal abrasion.
Accordingly, the present invention seeks to provide a composite forming fabric whose construction is intended at least to ameliorate the aforementioned problems of the prior art.
The present invention further seeks to provide a composite forming fabric having reduced susceptibility to cross-machine direction variations in the paper side layer mesh uniformity than comparable fabrics of the prior art.
Additionally, this invention seeks to provide a composite forming fabric that is resistant to lateral contraction.
This invention also seeks to provide a composite forming fabric that is more efficient to weave than comparable fabrics utilizing intrinsic weft binder yarns to interconnect the paper and machine side layer woven structures.
Furthermore, this invention seeks to provide a composite forming fabric that is less susceptible to dimpling of the paper side surface.
In a preferred embodiment, this invention seeks to provide a composite forming fabric having a lower void volume than a comparable forming fabric utilizing intrinsic weft binder yarns.
This invention additionally seeks to provide a composite forming fabric that is resistant to delamination.
In a first broad embodiment the present invention seeks to provide a composite forming fabric comprising in combination a paper side layer having a paper side surface, a machine side layer, and paper side layer intrinsic warp binder yarns which bind together the paper side layer and the machine side layer, wherein:
(i) the paper side layer and the machine side layer each comprise warp yarns and weft yarns woven together in a repeating pattern, and the paper side layer and the machine side layer together are woven in at least 6 sheds;
(ii) in the paper side layer all of the warp yarns comprise pairs of intrinsic warp binder yarns;
(iii) in the paper side surface of the paper side layer the repeating pattern provides a warp yarn path in which the paper side layer warp yarn floats over 1, 2 or 3 consecutive paper side layer weft yarns;
(iv) each of the pairs of intrinsic warp binder yarns occupy the unbroken warp path in the paper side layer;
(v) the ratio of paper side layer weft yarns to machine side layer weft yarns is chosen from 1:1, 2:1, 3:2, and 3:1; and
(vi) the ratio of paper side layer warp yarns to machine side layer warp yarns is chosen from 1:1 to 3:1; and wherein the pairs of intrinsic warp binder yarns comprising all of the paper side layer warp yarns are woven such that:
(a) in a first segment of the unbroken warp path:
(1) the first member of the pair interweaves with a first group of paper side layer wefts to occupy a first part of the unbroken warp path in the paper side surface of the paper side layer;
(2) the first member of the pair floats over 1, 2 or 3 consecutive paper side layer weft yarns; and
(3) the second member of the pair interlaces with one weft yarn in the machine side layer beside a machine side layer warp yarn that interlaces with the same machine side layer weft yarn;
(b) in a second segment of the unbroken warp path:
(1) the second member of the pair interweaves with a second group of paper side layer wefts to occupy a second part of the unbroken warp path in the paper side surface of the paper side layer;
(2) the second member of the pair floats over 1, 2 or 3 consecutive paper side layer weft yarns; and
(3) the first member of the pair interlaces with one weft yarn in the machine side layer beside a machine side layer warp yarn that interlaces with the same machine side layer weft yarn;
(c) in a third segment of the unbroken warp path:
(1) the first member of the pair interweaves with a third group of paper side weft yarns;
(2) the second member of the pair interweaves with the same third group of paper side weft yarns;
(3) both the first member and the second member each independently float over 1, 2 or 3 consecutive paper side weft yarns; and
(4) both the first member and the second member together occupy a third part of the unbroken warp path;
(d) in the paper side layer the unbroken warp path includes at least one first segment, at least one second segment, and at least one third segment, and at least one first or second segment is located between each of the third segments;
(e) the first, second and third segments are of equal or unequal length;
(f) the unbroken warp path in the paper side surface of the paper side layer occupied in turn by the first and the second member of each pair of intrinsic warp binder yarns has a single repeat pattern;
(g) in the unbroken warp path in the paper side surface of the paper side layer occupied in turn by the first and second members of each pair of intrinsic warp binder yarns, each succeeding segment is separated in the paper side surface of the paper side layer by at least one paper side layer weft yarn; and
(h) in the composite fabric the weave pattern of the first member of a pair of intrinsic warp binder yarns is the same, or different, to the weave pattern of the second member of the pair.
In a preferred embodiment of this invention, the fabric as woven and prior to heat setting has a warp fill of from 100% to 125%.
In further preferred embodiments of this invention, the fabric after heat setting has a paper side layer having an open area, when measured by a standard test procedure, of at least 35%, the fabric has a warp fill of from 110% to 140%, and the fabric has an air permeability, when measured by a standard test procedure, of from less than about 8,200 m3/m2/hr, to as low as about 3,500 m3/m2/hr at a pressure differential of 127 Pa through the fabric. An appropriate test procedure for determining fabric air permeability is ASTM D 737-96. Paper side layer open area is determined by the method described in CPPA Data Sheet G-18 using a plan view of this layer of the fabric.
It is a requirement of this invention that every paper side layer warp yarn comprises a pair of intrinsic warp binder yarns; each member of each pair alternately forms a portion of the unbroken warp path in the paper side surface weave pattern. Within each repeat of the composite fabric overall weave pattern, each paper side layer intrinsic warp binder yarn passes into the machine side layer to interlace at least once with a machine side layer weft, or wefts, so as to bind the paper side layer and the machine side layer together into a coherent composite fabric. The location at which each paper side layer intrinsic warp binder yarn interlaces with one machine side layer weft yarn is chosen to coincide with a knuckle formed by the interlacing of a machine side layer warp yarn with a machine side layer weft yarn.
In a preferred embodiment, within each repeat of the composite fabric weave pattern, at every machine side weft knuckle two warp yarns interlace with the machine side layer weft; one is a machine side layer warp, and the other is a paper side layer intrinsic warp binder yarn.
It can thus be seen that in the fabrics of this invention the paper side layer does not contain any conventional warp yarns which interlace only with paper side layer weft yarns. All of the paper side layer warp yarns are provided by the pairs of paper side layer intrinsic warp binder yarns, which, in addition to occupying the unbroken warp path in the paper side surface of the paper side layer, also bind the paper side layer and the machine side layer together.
Preferably, in the unbroken warp path in the paper side layer each segment occurs once within each complete repeat of the composite forming fabric weave pattern. Alternatively, in the unbroken warp path in the paper side layer each segment occurs more than once, for example twice, within each complete repeat of the composite forming fabric weave pattern.
Preferably, each first, second and third segment in the unbroken warp path in the paper side surface of the paper side layer is separated from an adjacent first or second segment by either 1, 2 or 3 paper side layer weft yarns. Preferably, each first, second and third segment in the unbroken warp path in the paper side surface of the paper side layer is separated from an adjacent first or second segment by one paper side layer weft yarn. Alternatively, each first, second and third segment in the unbroken warp path in the paper side surface of the paper side layer is separated from an adjacent first or second segment by two paper side layer weft yarns.
Preferably, within the paper side layer weave pattern, the first and second segment lengths formed by each of a pair of intrinsic warp binder yarns occupying the unbroken warp path are identical. Alternatively, the first and second segment lengths formed by each of a pair of intrinsic warp binder yarns occupying the unbroken warp path are not identical.
Preferably, within the composite fabric weave pattern the paths occupied by each of a pair of paper side layer intrinsic warp binder yarns are the same, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are regularly spaced, and are the same distance apart. Alternatively, within the composite fabric weave pattern the paths occupied by each of a pair of paper side layer intrinsic warp binder yarns are not the same, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are not regularly spaced, and are not the same distance apart. Preferably, within the composite fabric the weave design is chosen such that:
(1) the first, second and third segment lengths in the paper side layer are the same, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are regularly spaced; or
(2) the first, second and third segment lengths in the paper side layer are the same, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are not regularly spaced, and are not the same distance apart; or
(3) the first, second and third segment lengths in the paper side layer are not the same, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are not regularly spaced, and are not the same distance apart; or
(4) the first and second segment lengths in the paper side layer are the same, and are different from the third segment length, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are regularly spaced;
(5) the first and second segment lengths in the paper side layer are the same, and are different from the third segment length, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are not regularly spaced;
(6) the first and third segment lengths are the same, and are different from the second segment length, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are regularly spaced; or
(7) the first and third segment lengths are the same, and are different from the second segment length, and the interlacing points between the intrinsic warp binder yarns with the machine side layer wefts are not regularly spaced.
It is to be noted that within these preferred designs, both (6) and (7) are equally applicable when the second and third segment lengths are the same, and are different from the first segment length.
Preferably, the paper side layer weave pattern is chosen from a plain 1xc3x971 weave; a 1xc3x972 weave; a 1xc3x973 weave; a 1xc3x974 weave; a 2xc3x972 basket weave; a 3xc3x976 weave; a 4xc3x978 weave; a 5xc3x9710 weave; or a 6xc3x9712 weave. Preferably, the weave design of the machine side layer is an Nxc3x972N design such as is disclosed by Barrett in U.S. Pat. No. 5,544,678. Alternatively, the paper side layer may be combined with a machine side layer woven according to a satin, twill, or broken twill design.
Preferably, the ratio of the number of paper side layer weft yarns to machine side layer weft yarns in the composite forming fabric is chosen from 1:1, 2:1, 3:2 or 3:1.
Preferably, the ratio of paper side layer warp yarns to machine side layer warp yarns is either 1:1, 2:1 or 3:1, allowing for the fact that each intrinsic warp binder pair equates to a single paper side layer warp yarn. More preferably, the ratio is 1:1.
A composite forming fabric according to this invention will be woven to a pattern requiring from at least 6 sheds, and up to at least as many as 36 sheds. The number of sheds required to weave the composite fabric is equal to the number of sheds required to weave each of the paper side layer and the machine side layer designs within the overall pattern repeat of the composite fabric.
Generally, the number of sheds required to weave the paper side layer weave pattern will be an integral multiple of the number of sheds required to weave the machine side layer weave pattern. The value of the multiplier will be dependant upon the ratio of the number of paper side layer warps to machine side layer warps in the composite fabric. The number of sheds required to weave the paper side layer generally will be at least twice the number required to weave the machine side layer. This ratio can only be 1:1, that is, the same number of sheds to weave both the paper side layer and the machine side layer, when the machine side layer weave pattern is woven using twice the minimum number of sheds normally required. For example, if a 4-shed machine side layer weave pattern is woven in 8 sheds, the number of sheds to weave the paper side layer will be at least 8.
Table 1 summarizes some of the possible paper side layer and machine side layer weave pattern combinations, together with the shed requirements for each.
In the headings to Table 1, xe2x80x9cPSLxe2x80x9d indicates paper side layer, and xe2x80x9cMSLxe2x80x9d indicates machine side layer.
Because all of the pairs of intrinsic warp binder yarns making up the paper side layer warp yarns are utilized to interlace with machine side layer weft yarns, this interlacing pattern improves fabric modulus, thus making the composite fabric more resistant to stretching and distortion, while reducing lateral contraction and any propensity for fabric layer delamination.
An important distinction between prior art fabrics and those of the present invention is the total warp fill, which is given by warp fill=(warp diameterxc3x97meshxc3x97100)%. Warp fill can be determined either before or after heat setting, and, for the same fabric, is generally somewhat higher after heat setting. In all prior art composite fabrics, prior to heat setting, the sum of the warp fill in the paper side and machine side layers combined is typically less than 95%. The fabrics of this invention prior to heat setting have a total warp fill that preferably is greater than 100%, and is typically from 105% to about 125%. After heat setting, the fabrics of this invention have a total warp fill that preferably is greater than 110%, and is typically from about 110% to about 140%. This makes them unique. Another difference, associated with this level of warp fill, is that the mesh count of the paper side layer of the fabrics of this invention is at least twice that of the machine side layer. For example, one fabric of this invention was woven using 0.13 mm diameter warp and weft yarns to provide a paper side layer mesh (warpxc3x97weft) of 54.4xc3x9731.5 yarns/cm (counting each of the intrinsic warp binder yarn pair members); the machine side layer was woven using 0.17 mm diameter warp yarns and 0.33 mm diameter weft yarns to provide a machine side layer mesh of 27.2xc3x9715.75 yarns/cm. The resulting fabric had a total of 81.6 warp yarns/cm (54.4+27.2), and a total warp fill of 117% after heat setting.
In the context of this invention certain definitions are important.
The term xe2x80x9cunbroken warp pathxe2x80x9d refers to the path in the paper side layer, which is visible on the paper side surface of the fabric, of the pairs of intrinsic warp binder yarns comprising all of the paper side layer warp yarns, and which is occupied in turn by each member of the pairs making up the intrinsic warp binder yarns.
The term xe2x80x9csegmentxe2x80x9d refers to the portion of the unbroken warp path occupied by a specific intrinsic warp binder yarn, or by a pair of specific intrinsic warp binder yarns, and the associated term xe2x80x9csegment lengthxe2x80x9d refers to the length of a particular segment, and is expressed as the number of paper side layer weft yarns with which a member of a pair of intrinsic warp binder yarns interweaves, or both members of a pair of intrinsic warp binder yarns interweaves simultaneously, within the segment.
The term xe2x80x9cfloatxe2x80x9d refers to a yarn which passes over a group of other yarns without interweaving with them; the associated term xe2x80x9cfloat lengthxe2x80x9d refers to the length of a float, expressed as a number indicating the number of yarns passed over.
The term xe2x80x9cinterlacexe2x80x9d refers to a point at which a paper side yarn wraps about a machine side yarn to form a single knuckle, and the associated term xe2x80x9cinterweavexe2x80x9d refers to a locus at which a paper side yarn forms a plurality of knuckles with other paper side yarns along a portion of its length.